The invention is directed to a synchrotron for accelerating charged particles on a trajectory including straight portions, with which means for electron injection and acceleration as well as focusing are associated. The trajectory further contains curved portions with which superconducting curved flat coils are associated, which coils are arranged in a cryogenic vessel. In the curved portions, the trajectory is surrounded by chambers which are provided radially outwardly with at last one exit opening for synchrotron radiation.
As is well known, electrons and protons can be accelerated in a synchrotron to high energies by the provision that they are set in rotation on a curved trajectory and are repeatedly conducted through a high-frequency acceleration cavity. The particle always passes through the acceleration section if the applied a-c voltage has the sign which is correct for acceleration; the particle therefore revolves synchronously with the a-c voltage, i.e., with the correct phase. In an electron synchrotron, the electrons are introduced into the acceleration section already approximately at the speed of light; with the frequency of rotation being fixed, and only the electron energy remaining variable. The synchrotron radiation, i.e., the relativistic radiation emission of the electrons, which revolve approximately with the speed of light and are kept on a circular track by deflection in a magnetic field of superconducting coils, furnishes X-radiation with parallel radiation characteristics and high intensity. As is well known, this synchrotron radiation can be used for X-ray lithography which is suitable in the manufacture of integrated circuits, i.e. for generating structures which are smaller than 0.5 .mu.m. The parallel X-radiation strikes a mask to be imaged in the usable wavelength range of about .lambda.=0.2 to 2 nm. The semiconductor wafer to be exposed is arranged behind the mask at a suitable proximity spacing.
One known embodiment of an electron synchrotron contains a track in the form of a race track with alternatingly straight and curved track sections. The radius of curvature of the curved track sections is obtained by the equilibrium between the centrifugal force and the Lorentz force of a magnetic field of dipole magnets which are designed as superconducting curved flat coils. These field coils are arranged with a gradient coil in a cryogenic vessel which also keeps the evacuated chamber in the curved track section in which the electrons are circulated, at the cryogenic temperature. With the straight sections of the acceleration path are associated an electron injector, with which the electrons are introduced into the acceleration path, as well as means for accelerating the electrons (See, e.g., German Offenlegungsschrift No. 35 30 446).
In the above-described design for a synchrotron, the chamber is always provided with a slot-shaped exit opening extending along the entire curved track section of the track always. The Lorentz forces of the superconducting flat coils must therefore be taken up by the legs of a C-shaped section of a U-shaped support structure. Since a change in position of the flat coils under the action of the Lorentz forces must be avoided to prevent a corresponding field distortion, a correspondingly elaborate support structure is necessary.
It is therefore an object of the invention to simplify and improve the support structure for the field coils of the dipole magnets in the curved region of the track; and in particular, to provide a simplified structure to prevent bending stresses in the legs of the C-shaped sections.
According to the invention, the object is achieved by providing an absorber in each of the chambers surrounding the electron trajectory. The absorber leaves free for the synchrotron radiation at least one and optionally several exit openings which are preferably designed as exit tubes. The space between these tubes in the direction of the tangentially conducted-off synchrotron radiation behind the absorber can now be filled with a support structure, for example, support elements preferably comprising fiber glass-reinforced plastic GFK. By virtue of the support structure, which in practice acts only as a simple spacer, large magnetic forces of the superconducting coils can be taken up such that a special support structure is no longer required.
To limit the heating of the walls of the electron beam chamber, which is kept at cryogenic temperatures, as well as to reduce the desorption of particles of the material of the absorber, it is advisable to provide additional cooling for the absorber.
For a further explanation of the invention, reference should be made to the following detailed description and the accompanying drawings, in which an exemplary embodiment of a synchrotron according to the invention is illustrated schematically.